(1) Field of the Invention
This invention relates to bipolar, filter press type electrochemical cells having porous, self-draining electrodes and liquid permeable diaphragms.
(2) Description of the Prior Art
Packed bed chlor-alkali electrolytic cells are known from Oloman et al U.S. Pat. No. 3,969,201 and U.S. Pat. No. 4,118,305. Improvements in these cells have been disclosed by McIntyre et al in U.S. Pat. No. 4,406,758; U.S. Pat. No. 4,431,494; U.S. Pat. No. 4,445,986; U.S. Pat. No. 4,511,441; and U.S. Pat. No. 4,457,953. These packed bed electrolytic cells are particularly useful for the production of alkaline solutions of hydrogen peroxide.
Among known bleaching agents, hydrogen peroxide is at the present time being increasingly used, in particular for bleaching materials such as textiles or paper pulp. Hydrogen peroxide has the great advantage over other bleaching agents, in particular chlorine and its compounds, in that because of its mild action, it attacks the fibers of the material to be treated to a much lesser extent and gives a better finish.
Hydrogen peroxide is generally used in bleaching in the form of a stabilized alkaline solution of low peroxide concentration. The action of hydrogen peroxide in bleaching consists essentially of destroying or decolourising the natural dyes by oxidation, or by rendering them soluble. Even though the mechanism of these reactions has been little studied, it is generally assumed that the hydrogen peroxide ion HOO.sup.- is responsible for the bleaching.
Present-day bleaching solutions based on hydrogen peroxide have the great disadvantage with respect to other conventional bleaching solutions (in particular hydrochlorite-based solutions) of being relatively costly Their widespread use is very dependent upon economic considerations, particularly when large quantities of low-value material such as paper pulp are to be treated Present-day bleaching solutions are nearly always prepared by simple dissolving and dilution, starting from commercially available chemicals. Commercially available hydrogen peroxide is a particularly costly substance, as it is manufactured only in a small number of large industrial plants, and it has therefore to be highly concentrated for storage and transport purposes before being distributed. At the present time there is a need to replace the highly concentrated, commercially available hydrogen peroxide by on site manufacturing methods which enable dilute solutions of hydrogen peroxide to be produced directly, in order to reduce bleaching costs. However, up to the present time no satisfactory method has appeared.
Hydrogen peroxide is used not only for bleaching purposes, but also in an increasing number of other processes, in particular in the pollution control field. However, treatment solutions used for this purpose are likewise almost always prepared from highly concentrated hydrogen peroxide with the same disadvantages as heretofore stated.
In Grangaard, U.S. Pat. No. 3,607,687; U.S. Pat. No. 3,462,351; U.S. Pat. No. 3,507,769; and U.S. Pat. No. 3,592,749 there are disclosed electrolytic cells for the production of hydrogen peroxide in which the peroxide is produced in the cathode compartment of the cell which contains a cathode depolarized utilizing an oxygen containing gas. The electrochemical cells of Oloman et al and McIntyre et al disclosed in the patents cited above, are improvements over the cells of Grangaard partly as the result of the use of the novel electrode material disclosed in U.S. Pat. No. 4,457,953 in which there is disclosed a method for the production of coated particles for use in a packed bed electrode electrochemical cell.
It has been found that a packed bed, self-draining cathode for maximum productivity within an electrochemical cell for the production of an alkaline hydrogen peroxide solution must be supplied with a liquid anolyte through an electrolyte porous diaphragm at a substantially uniform rate of flow across the porous diaphragm without appreciable variation of the flow rate as a function of the head of the electrolyte. Prior art porous diaphragms for packed bed electrolytic cells have permitted a considerable variation in flow rate with the flow rate at the base of the cell (exposed to the full head of the electrolyte) being appreciably faster than the flow rate in the center of the cell or at the top of the cell, where a decreased head pressure is exerted on the diaphragm. This variation in flow rate has resulted in inefficiency of the cell. Where an attempt has been made to reduce the flow rate through the cell diaphragm, it has been found that too little electrolyte passes through the porous diaphragm into the cathode where the diaphragm is exposed to a minimal head of electrolyte. A reduced amount of electrolyte passing through the porous diaphragm into the cathode also results in an increase in cell voltage. An excessive amount of electrolyte passing through the porous diaphragm causes flooding of the packed bed cathode and consequent reduction in the depolarizing effect of the oxygen containing gas fed to the side of the packed bed cathode opposite to that which is exposed to the electrolyte.